It's rarely used anymore. As ondkaja mentioned, it was used when LAN centers barely had any desk space. Cooller doesn't use it anymore, and he's probably one of the first people that really made it known.
There's no real advantage to using it, it's just preference (and again, if you lack desk space). I used it for a little while, and maybe 1-2 LANs where I didn't have much space. It kinda hurt my wrist after a while, and typing was very inconvenient. It won't make you better, and any belief where you think it is making you play better is purely psychological.